I suspect that in all likelihood, Data is simply under the false impression that he's incapable of forming contractions, much like he mistakenly accepts that he was created completely without emotion, when they constantly flirted with the contrary.

Data was obviously capable of very simple, less broad, emotions. He may not have been capable of feeling love, humor or sadness but he had curiosity. Which can be an emotion, he had desires and wants. Both can be emotions. He had some degree of "affection" for Spot, he attachment to his friends and even "missed" them when absent. So while he may not have been able to "feel sad" or "feel happy" he had "emotion" in some manner or another.

What's even odder Data seemed to have a built-in system to NOT improve himself or achieve goals. When he thinks Q is going to grant him with humanity at the end of (Deja Q?) Data is pretty quick to dismiss it. Same with when Riker -with the power of the Q, was going to give Data that gift. Data turned it down simply because he didn't want to have to "thank" Q for it. What kind of logic is that? Something in Data wanted to be as human as possible so he strived to achieve as much as possible but innately within him he DIDN'T want to be human!